Solvent effects vibrational spectroscopy

The use of computer simulations to study internal motions and thermodynamic properties is receiving increased attention. One important use of the method is to provide a more fundamental understanding of the molecular information contained in various kinds of experiments on these complex systems. In the first part of this paper we review recent work in our laboratory concerned with the use of computer simulations for the interpretation of experimental probes of molecular structure and dynamics of proteins and nucleic acids. The interplay between computer simulations and three experimental techniques is emphasized (1) nuclear magnetic resonance relaxation spectroscopy, (2) refinement of macro-molecular x-ray structures, and (3) vibrational spectroscopy. The treatment of solvent effects in biopolymer simulations is a difficult problem. It is not possible to study systematically the effect of solvent conditions, e.g. added salt concentration, on biopolymer properties by means of simulations alone. In the last part of the paper we review a more analytical approach we have developed to study polyelectrolyte properties of solvated biopolymers. The results are compared with computer simulations. 

In our discussion the usual Born-Oppenheimer (BO) approximation will be employed. This means that we assume a standard partition of the effective Hamiltonian into an electronic and a nuclear part, as well as the factorization of the solute wavefunction into an electronic and a nuclear component. As will be clear soon, the corresponding electronic problem is the main source of specificities of QM continuum models, due to the nonlinearity of the effective electronic Hamiltonian of the solute. The QM nuclear problem, whose solution gives information on solvent effects on the nuclear structure (geometry) and properties, has less specific aspects, with respect the case of the isolated molecules. In fact, once the proper potential energy surfaces are obtained from the solution of the electronic problem, such a problem can be solved using the standard methods and approximations (mechanical harmonicity, and anharmonicity of various order) used for isolated molecules. The QM nuclear problem is mainly connected with the vibrational properties of the nuclei and the corresponding spectroscopic observables, and it will be considered in more detail in the contributions in the book dedicated to the vibrational spectroscopies (IR/Raman). This contribution will be focused on the QM electronic problem. 

More recently, the PCM has been amply extended to the treatment of vibrational spectroscopies, by taking into account not only solvent-induced vibrational frequency shifts, but also vibrational intensities in a unified and coherent formulation. Thus, models to treat IR [8], Raman [9], IR linear dichroism [10], VCD [11] and VROA [12] have been proposed and tested, by including in the formulation local field effects, as well as an incomplete solute-solvent regime (nonequilibrium) and, when necessary, by extending the model to the treatment of specific solute-solvent (or solute-solute) effects. 

Solvent effects on vibrational spectroscopies are analyzed by Cappelli using classical and quantum mechanical continuum models. In particular, PCM and combined PCM/discrete approaches are used to model reaction and local field effects. 

The second chapter ends with two overviews by Stephens Devlin and by Hug on the theoretical and the physical aspects of two vibrational optical activity spectroscopies (VCD and VROA, respectively). In both overviews the emphasis is more on their basic formalism and the gas-phase quantum chemical calculations than on the analysis of solvent effects. For these spectroscopies, in fact, both the formulation of continuum solvation models and their applications to realistic solvated systems are still in their infancy. 

PCM originated as a method to describe solvent effects on ground state molecules [2], but the extension to excited states was realized only after the original presentation, with a model [3], which introduced nonequilibrium effects in the solvent response for the optical processes of photon absorption and emission. The nonequilibrium solvation regime has later been applied to vibrational spectroscopies... 

What is not discussed at length are photophysical phenomena in SCF solvents (e.g. fluorescence quenching, triplet-triplet annihilation, charge transfer, and exiplex formation) which have been extensively used to probe SCF properties, in general, and have been especially informative regarding the existence of clusters (solvent-solute and solute-solute) and their effect on reactivity. Absorption and fluorescence spectroscopy (both steady state [21-33] and time resolved [34-40], vibrational spectroscopy [41-44] pulse radiolysis, [45] and EPR [46,47] have all been utilized in this regard. 

Continuum Solvation Models Efficient Way to Account for Solvent Effects in Vibrational Spectroscopy... 

Solvation Regime When dealing with solvent effects, it is to be kept in mind that absorption and emission are time-dependent processes. In this case (as ours), the time dependency of the spectroscopic phenomenon is not explicitly accounted for in the computational methodology, that is, time-independent methods are exploited. Moreover, the motions associated to the degrees of freedom of the solvent molecules involve different time scales. In the particular case of vibrational spectroscopy, typical vibration times being on the order of lO -lO s, it is clear that the orientational component of the solvent polarization cannot instantaneously readjust to follow the oscillating solute, so that a nonequilibrium solute-solvent system should in principle be considered. 

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